1. Field of the Invention
The present invention relates to a mobile communication system for providing communication services to user equipments, the user equipment in the mobile communication system, a control program thereof, and a transmission power control method in the mobile communication system.
2. Description of the Related Art
In recent years, a mobile communication system using W-CDMA (Wideband-Code Division Multiple Access) has been used as a multiple access method.
In the mobile communication system using the W-CDMA (hereinafter appropriately referred to as “W-CDMA wireless system”), transmission power control is performed for maintaining a communication quality to a predetermined level.
Moreover, in various environments, in order to more stably maintain the communication quality, closed-loop transmission power control having a double-loop structure consisting of inner loop control and outer loop control may be performed as the transmission power control.
FIG. 3 is a schematic diagram showing a concept of the closed-loop transmission power control having the double-loop structure.
As shown in FIG. 3, in the closed-loop transmission power control having the double-loop structure, in the inner loop control, an instruction for causing a transmission power to increase or decrease is issued with respect to an opposite base station so that a Signal to Interference Ratio of a received radio signal (received SIR) in a user equipment (UE) matches a received SIR to be targeted (target SIR). Moreover, in the outer loop control, the target SIR in the inner loop control is caused to increase or decrease so that a received Block Error Rate (received BLER) of user data in the user equipment matches a received BLER to be targeted (target BLER).
Here, in a stand-alone communication state (StandAlone-DCCH state) using Blind transport format detection as is prescribed in 3GPP 25.212 Annex-A, a state occurs in which only a DCCH is set and a DTCH is not set.
Specifically, when a call button is depressed on the user equipment in an IDLE state, a state occurs in which Radio Resource Control (RRC) has been established in a connection procedure performed between the user equipment and the base station. At this time, only the DCCH has been set as a dedicated physical channel, and the DTCH has not been set in this state. Moreover, the user equipment performs the Blind transport format detection with respect to the DCCH.
Next, in a subsequent connection procedure, Radio Access Bearer (RAB) is established.
Then, in addition to the DCCH, the DTCH is set and the connection procedure is completed.
In this connection procedure, the state in which only the DCCH is set and the DTCH is not set is the above described stand-alone communication state.
In this stand-alone communication state, since a CRC (Cyclic Redundancy Check) is not transmitted, the BLER in the received signal cannot be calculated. In other words, the user equipment performs a process of constantly waiting for data, and if a result of the CRC is OK, determining that there is the data, and decoding received data. Therefore, if the DTCH has not been set, the result of the CRC cannot be determined. Consequently, also the BLER cannot be measured.
Thus, a situation occurs in which the above described outer loop control for matching the received BLER with the target BLER cannot be performed.
With respect to such a situation, as a method of determining the target SIR in the stand-alone communication state, there have been proposed a method of fixedly using a particular target SIR value which has been previously defined, in a StandAlone-DCCH period (see JP2003-318818A), and a method of determining the target SIR based on a received data error rate of a pilot signal in a dedicated physical control channel (DPCCH) (see JP2004-274117A).
However, if the fixed value which has been previously defined as the target SIR in the outer loop control is used, since the target SIR is not changed, a power more than or equal to a certain level is not requested even though a reception quality has deteriorated, and thereby a situation occurs in which data transmission and reception are failed. Conversely, an excessive transmission power is requested even though the reception quality has been improved, and thereby a situation occurs in which power consumption is increased or interference to another communication is caused.
Moreover, if the target SIR is determined based on the received data error rate of the pilot signal in the dedicated physical control channel, since the pilot signal in the dedicated physical control channel (DPCCH) is discretely transmitted and an information amount is limited, the target SIR to be set may not be an appropriate value.
Here, as a method of updating the target SIR in the StandAlone-DCCH period by the outer loop control, a method of transmitting a signal attached only with the CRC from the base station to the user equipment and measuring the BLER of a dedicated channel even if there is no control information is conceivable. However, the signal is transmitted even though there is no control information, which causes unnecessary power consumption, and also, although a CRC check has been completed at one time, the number of CRC checks increases twice in the Blind transport format detection under a condition of the StandAlone-DCCH.
In this way, it has been difficult to perform adequate transmission power control depending on a communication status if the Blind transport format detection is used in the stand-alone communication state.